1. Field of the Invention
The present invention relates to system for controlling the air-fuel ratio of an internal combustion engine, or more in particular to an air-fuel ratio control system suitably used with an internal combustion engine for automobiles equipped with exhaust gas purification means including an air-fuel ratio sensor and a three-way catalyst.
2. Description of the Related Art
An air-fuel ratio feedback (F.B.) control using an oxygen gas sensor is practically used as a conventional means against exhaust gas. Such a control system greatly improves the accuracy of the air-fuel ratio control. However the speed of the internal combustion engine fluctuates in synchronism with the cycles of change in the compensation of the feedback control at the time of engine idling, thus causing an uncomfortable feeling to the driver. To obviate this problem, a prior art control system (such as disclosed in JP-A-58-217745) has been designed to stop the integrating processing of the compensation amount and holding the compensation amount at a predetermined value for a predetermined period of time when the internal combustion engine shifts to idling state.
The above-mentioned conventional method of control, however, is so constructed that the compensation amount is skipped upward from a small value in the non-idle state, the resulting compensation amount is held for a predetermined time, the compensation amount is then increased stepwise little by little, and when the air-fuel ratio shifts from lean to rich side, the compensation amount is reduced slightly and is held for a predetermined length of time. As a consequence, the air-fuel ratio tends to be on lean side, thereby making it impossible to control the air-fuel ratio properly